In known generators of this type, the excitation current through the exciter winding is derived from the main current in the generator; for some AC generators by means of separate exciter diodes. When the generator starts up, self-excitation takes place because of the small remanent voltage in the generator. The latter results from the fact that residual magnetism in the iron core causes a small electromotive force to be generated in the winding. This small electromotive force (emf) generates a small current in the closed circuit of the exciter winding so that a small amount of electromagnetism is added to the remanence, reinforcing its effect. The emf thus continually increases until the desired emf corresponding to the rotary speed of the generator has been reached. This takes place within a very short time.
This type of self-excitation is decreased by the resistance of the regulator circuit connected in series with the exciter winding. The regulator circuit may, for example, be the emitter-collector circuit of the output transistor of the current regulating circuit. Other circuit elements connected in series with the exciter winding are, for example, the exciter diodes and, for some AC generators, the so-called minus diodes, that is the diodes connected to the negative DC output terminal. Since the main control circuit of the regulator is connected in series with the exciter of field winding of the generator and since the voltage across this regulator circuit may be as much as 1.5 volts, either external excitation of the AC generator must be supplied during start-up or the generator will deliver a useful output only after reaching a relatively high speed. This is a distinct disadvantage since modern AC generators in motor vehicles should be able to supply a charging current even at idling speeds. Further the possibility exists that excitation by means of the battery may not be possible.